1. Field of the Invention
The present invention relates generally to high-voltage varactor diodes.
2. Description of the Related Art
In a semiconductor, charge carriers (electrons or holes) diffuse from a high carrier-density region to a low carrier-density region. For this reason, charge carriers diffuse across the junction of an unbiased semiconductor diode to create a depletion region of ionized atoms, i.e., atoms which have lost their mobile carriers. Once this built-in potential V.sub.bi has been established by the initial diffusion, it acts as a barrier to further diffusion.
If a reverse bias is imposed across the diode, the depletion region widens to expose a region of negative charges (due to acceptor atoms) on one side of the junction and a region of positive charges (due to donor atoms) on the other. The width of the depletion region is a function of the impurity doping levels of the diode junction. If both sides of the junction are equally doped, the depletion region will extend an equal distance from the junction. With unequal doping levels, the depletion region will extend farther into the side which has the smaller impurity concentration.
The electric field is found by integrating the negative and positive charges. In contrast, the potential drop across the junction is found by a second charge integration or, equivalently, an integration of the electric field. If the doping concentration is constant, the electric field in the depletion region peaks at the junction and decreases linearly to the edges of the depletion region. In this case, the potential drop across the depletion region has a quadratic form.
If the reverse bias is increased to a breakdown voltage V.sub.BR, a large reverse current results because the electric field at the junction exceeds the dielectric strength of the diode's semiconductor material. Covalent atomic bonds are ruptured, a large number of minority carriers are released and the diode is said to avalanche. The electric field and potential drop of depletion regions has been developed by many authors (e.g., Singh, Jasprit., Semiconductor Devices, McGraw-Hill, Inc., New York, 1994, pp. 192-208). In contrast with diodes that are purposely intended to operate in breakdown (e.g., see microwave IMPATT diodes in Sze, S. M., High-Speed Semiconductor Devices, Wiley-Interscience Publications, New York, 1990, pp. 539-544), varactor diodes are generally configured to avoid breakdown over an operational reverse-bias range.
In a varactor diode, each side of the diode junction is conductive and the depletion region acts as a dielectric so that a reverse-biased semiconductor junction has the structure of a capacitor, i.e., two conducting regions separated by a dielectric. The capacitance depends directly on the junction area and inversely on the width of the depletion region, i.e., C=(.epsilon.A)/d in which s is the dielectric constant, A is the junction's cross-sectional area and d is the width of the depletion region. The diode capacitance decreases with increased reverse bias because this change in bias causes the depletion width to increase. The capacitance ratio over a specified reverse bias range is generally referred to as the tuning ratio. These and other varactor diode basics are described in a variety of sources (e.g., Norwood, Marcus H., et al., Voltage Variable Capacitor Tuning: A Review, Proceedings of the IEEE, Vol. 56, No. 5, May, 1968).
Varactors find utility in a variety of electronic circuits. For example, a varactor diode in a resonant circuit can control the frequency of a voltage-controlled oscillator (VCO) or the amplifier frequency in a receiver. Typically, VCOs and receiver amplifiers are tuned smoothly across their operating bands. Accordingly, varactor diodes for these applications usually exhibit a capacitance that is proportional to an exponential power of the reverse-bias voltage V.sub.r, e.g., C.varies.(V.sub.r).sup.-0.5. This exponential relationship between capacitance and bias voltage is a feature of abrupt and hyperabrupt varactor diodes. Abrupt-junction diodes have uniform doping on each side of the junction with an abrupt transition at the junction. In hyperabrupt-junction diodes, the doping level increases as the junction is approached from either side. A detailed description of these varactor diodes can be found in numerous references (e.g., Bogart, Theodore F., Electronic Devices and Circuits, MacMillan Publishing Company, New York, 1993, pp. 891-893).
In contrast, some applications require varactor diodes whose capacitance is an irregular function of bias voltage. For example, power electronic systems develop voltage and current forms as required by user loads. They often include serially-connected resonant converters which utilize switching circuits to achieve high power-conversion efficiencies. To reduce switching stresses and excess production of electromagnetic interference, each switch is preferably switched by control and timing circuits when the switch's voltage and/or current is at a minimum. The design of the control and timing circuits would be facilitated by the availability of a high-breakdown varactor diode that exhibited a bi-level capacitance characteristic.
In particular, a preferred varactor diode for these switching applications has the following characteristics relative to an applied reverse bias V.sub.r : (a) a high reverse breakdown V.sub.BR, e.g., preferably&gt;100 volts, (b) a large tuning ratio of maximum capacitance C.sub.max to minimum capacitance C.sub.min, e.g., preferably C.sub.max /C.sub.min &gt;2:1, (c) a rapid transition between C.sub.max and C.sub.min in the region of a transition voltage V.sub.TR, (d) the transition voltage V.sub.TR occurs between a selected minimum V.sub.TR(min) and a selected maximum V.sub.TR(max), (e) the maximum capacitance C.sub.max is substantially constant for 0&lt;V.sub.r &lt;V.sub.TR, and (f) the minimum capacitance C.sub.min is substantially constant for V.sub.TR &lt;V.sub.r &lt;V.sub.BR.The high V.sub.BR and the high C.sub.max /C.sub.min parameters are particularly desirable characteristics of the preferred varactor diode.
Presently vailable varactor diodes do not exhibit the preferred capacitance/voltage characteristics. For example, MA46400 and MA46600 gallium arsenide, hyperabrupt varactors from M/A-COM Semiconductor Products of Burlington, Mass. have a large tuning ratio but low breakdown voltages, e.g., V.sub.BR &lt;50 volts and MA45200 silicon abrupt varactors from M/A-COM Semiconductor Products have high breakdown voltages, e.g.,.about.90 volts, but low tuning ratios in the required transition region.